The present invention relates to fluidized bed combustors and the like and, more particularly, to a heat exchanger feed system and method for distributing fuel through the walls of a combustor or other heat exchanger.
The use of fluidized bed combustors has long been recognized as an attractive way of generating heat. In these arrangements, air is passed through a bed of particulate material, including a fuel, such as coal, for burning and an adsorbent, such as limestone, for adsorbing the sulfur generated as a result of the burning, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. The heat produced by the burning of the fuel in the fluidized bed is utilized in various ways such as to generate electricity and to provide marine propulsion. The fluidized bed has become a preferred system of generating heat in combustors since it affords an attractive combination of high heat release, high sulfur adsorption, low nitrogen oxides emissions and a reduction in combustor size. A further advantage of fluidized bed combustors has been their fuel flexibility, however this advantage has been technologically limited by the design of the fuel feeding systems as described below.
In fluidized bed combustors, the particulate fuel material must be continuously, or at least periodically, distributed into the bed to replenish the spent fuel expended in the combustion process. It has been suggested to provide in-bed feeding systems in which the particulate fuel material is introduced directly into the bed from a point below the bed's upper surface. These systems, however, require a multiplicity of feed points to prevent hot spots or over-cool spots from forming in the bed since the lateral transfer or dispersion of the materials through the bed is relatively poor.
Therefore, many conventional fluidized bed combustors utilize a feeder, or feeders, which distribute the particulate fuel material from a position above the upper surface of the bed where it falls onto the bed by gravity. Pivotally mounted distributor trays can be used in connection with such above-bed feeders to insure a uniform distribution of the material across the upper surface of the bed as is shown in U.S. Pat. No. 4,275,668, a patent assigned to the same assignee as the present application.
These above-bed feeders typically use downspouts to deliver the material into the combustor. These downspouts must be sloped at relatively shallow angles to the horizontal to clear structures on the outside walls of the combustor such as buckstays, insulation and lagging, as well as to allow for expansion. These shallow angles, however, limit the types of materials which can be delivered through above-bed feeders to those with relatively small cohesive strengths. When materials with high cohesive strengths are delivered, the downspouts become clogged and the material flows unevenly due to the high friction forces which develop in shallow angled chutes. Therefore, even though fluidized bed combustors are able to efficiently consume many different types of fuels, no current technology exists to economically deliver all of these fuels into the combustor.